This invention relates generally to RF filter circuits and is particularly directed to a tunable notch filter for the front end of an RF receiver for reducing conducted local oscillator leakage and improving radio frequency image rejection.
A conventional radio frequency signal receiver includes a radio frequency (RF) section and an intermediate frequency (IF) section. Included in the RF section are tuned RF filters to provide coarse filtering for a band of frequencies centered about a selected channel. The output of the RF filters is provided to an RF amplifier which typically has a gain of at least 20 to 30 dB as in a conventional television receiver. The output of the RF amplifier is provided to one input of a mixer circuit, while a second input of the mixer receives a signal from a local oscillator which is offset in frequency from the selected channel by a given amount for generating a heterodyned lower, intermediate frequency (IF) signal. In a television receiver, the IF signal frequency is approximately 45 MHz. The output of the mixer is filtered and amplified in an IF stage.
While the heterodyning of the selected RF signal and the local oscillator generated signal in the mixer thus produces the desired 45 MHz IF signal, an undesired 45 MHz signal may also be produced in response to the heterodyning of the local oscillator signal with a received RF signal having a frequency 90 MHz higher than the selected frequency (commonly referred to as the image frequency). This undesired 45 MHz component manifests itself as a spurious beat in the processed video signal and degrades the resulting video image. In addition, the local oscillator signal is frequently provided via the mixer to not only the IF stage, but also back to the antenna resulting in unwanted transmissions from the receiving antenna in violation of federal communications regulations.
The present invention is intended to eliminate the aforementioned problems in a wide band RF receiver by providing a tunable notch filter for image frequency rejection and conducted local oscillator leakage reduction for use in the interstage portion of the receiver's front end. In response to a tuning voltage provided thereto, the tunable notch filter is not only tuned to a desired RF frequency, or channel, but also is de-tuned to a frequency approximately equal to that of the local oscillator so as to exhibit a notch in its frequency response thereat.